Fluorescent lamps are well known light sources famous for their high light output and relatively long life. Such lamps comprise a tubular, hermetically sealed, glass envelope having electrodes sealed in the ends thereof. An arc generating and sustaining medium, usually at low pressure, and comprising one or more inert gases such as argon, krypton, etc., or mixtures thereof, together with a small amount of mercury, is present in the envelope. The interior of the envelope is coated with a layer of phosphor which will absorb various forms of energy generated by the arc (usually wavelengths of ultraviolet) and reemit this energy in the form of visible light.
Those lamps, as well as all other known lamps, suffer from a gradual decrease in light output as they age. The light output of a lamp at any time is given as a fraction or a percentage of the original output and is called the maintenance at that time. Maintenance can be measured in lumens or other arbitrary units. Poor maintenance has been a major factor preventing the successful application of many phosphors.
The conditions that cause the loss in light output are many and include the initial processing conditions where the lamp is baked to temperatures of 600.degree. C. which can cause serious degradation in the performance of some phosphors.
After completion of the lamp, during operation thereof, the phosphor is subjected to the mercury vapor discharge where it is bombarded by ions as well as being exposed to high energy ultraviolet 185 nm and 254 nm radiation. Studies have shown that the 185 nm radiation is considerably more detrimental to alkaline earth halophosphate phosphors than the 254 nm radiation.
A number of techniques have been suggested to overcome or at least retard the decrease in loss of light output. These techniques have included better processing of the phosphors, and methods to shield the phosphors from the deleterious effects of the lamp processing and arc discharge by the application of a protective film over the phosphor. Various materials for this shielding have included non-continuous particulate films of, for instance, silica and alumina.
One technique to improved maintenance is described in U.S. Pat. No. 4,459,507, dated July 10, 1984 issued to J. M. Flaherty and assigned to the Assignee of the present application. This patent involves applying a non-luminescent maintenance improving film of yttrium oxide overlying the phosphor. This film protects the phosphor from ion bombardment but not from the deleterious effects caused by the 185 nm radiation.
While all of the above techniques have provided an improvement in the maintenance, it would be an advance in the art to further improve maintenance as well as increase the total integrated light output of fluorescent lamps.